ASSESSMENT OF THE DIGGING FORCE FOR UNDERWATER COAL MINING

Abstract

Lack of geomechanical parameters for the digging force in underwater mining conditions was noticed during analyzing the available technical documentation at preparatory phase for the revitalization of the vessel bucket wheel excavator "Kovin" vital parts. This fact has caused the need for the implementation of relevant research program that will determine the water pressure effect on the coal mining process. The research program included certain geomechanical laboratory tests which are adapted to the specific conditions and needs of underwater coal mining. The program had included coal laboratory testing of the linear and surface digging force in underwater conditions at different hydrostatic pressures of the water column. Intention of this laboratory testing program was to simulate underwater coal mining conditions at different depths of the excavation-digging according to the "in situ" principle.

Full text

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7th BALKANMINE CONGRESS – Proceedings, Prijedor 2017
DOI: 10.7251/BMC170701279C
ASSESSMENT OF THE DIGGING FORCE FOR
UNDERWATER COAL MINING
Vladimir ĈEBAŠEK1, Nebojša GOJKOVIĆ1, Zvonimir BOŠKOVIĆ2,
Bojan DIMITRIJEVIĆ 1, Veljko RUPAR1
1University of Belgrade, Mining and Geology Faculty, Serbia, vladimir.cebasek@rgf.bg.ac.rs
nebojsa.gojkovic@rgf.bg.ac.rs, bojan.dimitrijevic@rgf.bg.ac.rs, veljko.rupar@rgf.bg.ac.rs
2University of Bana Luka, Mining Faculty, Prijedor, Republic of Srpska, BiH, zvonimir.boskovic@rf.unibl.org
ABSTRACT
Lack of geomechanical parameters for the digging force in underwater mining conditions was noticed
during analyzing the available technical documentation at preparatory phase for the revitalization of
the vessel bucket wheel excavator "Kovin" vital parts. This fact has caused the need for the
implementation of relevant research program that will determine the water pressure effect on the coal
mining process. The research program included certain geomechanical laboratory tests which are
adapted to the specific conditions and needs of underwater coal mining. The program had included
coal laboratory testing of the linear and surface digging force in underwater conditions at different
hydrostatic pressures of the water column. Intention of this laboratory testing program was to simulate
underwater coal mining conditions at different depths of the excavation – digging according to the "in
situ" principle.
Key words: digging force, underwater mining, coal
1. INTRODUCTION
Continuous technology is used for overburden, gravel and coal exploitation on this
deposit. Overburden and gravel excavation is carried out with the vessel bucket-chain
excavator "5630", the remaining quantities of gravel and sand, which are not within the reach
of the vessel bucket-chain excavators, as well as coal are excavated by means of the vessel
bucket-wheel excavator model UCW 450, so called "Kovin" [1]. The analysis of the available
documentation necessary for the revitalization of the vital parts of the vessel bucket-wheel
excavator "Kovin", showed the lack of geomechanical parameters which necessary to
determine the influence of the aquatic environment pressure on the work environment - coal,
and it promoted the necessity for appropriate research of the water column pressure influence
to the digging force value.

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2. GEOLOGICAL STRUCTURE OF COAL AND GRAVEL DEPOSITS, KOVIN
(COAL DEPTH)
Coal and gravel deposit "Kovin" represents the northern part of the unique sedimentation area
Kostolac-Kovin, which is separated by the Danube river. Gravel deposit stratigraphically
belongs to the pleistocene, and it was developed in the riverbed facies. Its evolution is being
continuously monitored at the entire exploration area. Kovin coal deposit belongs to the
stratigraphic unit of the upper miocene or pontian actually his younger part - top pontian. In
this area of the substratum section of the miocene part are pontian and pannonian formations,
while overlying strata corresponds to quaternary formations [1], Figure 1.
3. WATER PRESSURE ON THE MINING LEVEL
The conditions under which it is necessary to determine the value of digging force should as
much as possible to simulate the conditions that are present during the process of the
excavation "in-situ" and they are determined on the basis of the depth analysis at which the
coal exploitation is carried out. The maximum depth of which is necessary to perform the test
is determined on the basis of the fact that the coal layer is characterized by frequent thickness
change, and for safety instead of the second coal layer average depth of 50 m, maximum
depth of 60 m was employed.
Figure 1. Schematic representation of the geological coal and gravel deposit "Kovin"
with showed pressure changes with increasing depth
The conditions that exist in the process of coal excavation in this deposit were analyzed for
the steady water (fluid) state, which may be subjected to the basic laws of hydrostatics as part
of fluid mechanics. In any liquid that is under influence of the earth's gravity, pressure within
itself creates, and it does not depend on the amount of fluid in the vessel, but on the height of
the liquid column. The foregoing implicates that with increasing of observed point depth in
the fluid there is an increase of liquid weight which is located above this point, and therefore
the pressure increases. In order to determine the pressure values at the given point on a
particular depth in the liquid it is necessary to determine the hydrostatic pressure using
Torricellis formula (Evangelista Torricelli, 1608-1647):
hgP 

where: ρ – liquid density [kg/m3],
g – gravity acceleration [m/s2],
h – depth [m].

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If product of the liquid density (ρ) and the gravitational acceleration (g) is replaced by a unit
weight (γ) the hydrostatic pressure equation will be in the following form:
hP 

where: γ – liquids unit weight (kN/m3),
h – depth (m).
Hydrostatic pressure change with the observed point depth increase, in this case point at
which coal is excavated, is graphically shown in Figure 2.
Hydrostatic pressure in the given point at a certain depth in the fluid acts equally in all
directions. Considering the structural and physical properties, particularly fracturing and
porosity, in the wider zone of the excavation a coal layer is fully saturated with water. The
water influence, in terms of external loads, can be defined as the coal is immersed in the
liquid at depth at which excavation is carried out, whereby the pressure of the water column,
according to the law of hydrostatics acts uniformly in every direction, Figure 2.
Figure 2. Influence of water on the coal during the excavation
4. METHODOLOGY TESTING OF DIGGING FORCE
Specific digging force for rock material (coal) was determined using the test methods
supplemented with a wedge that has been proposed by the company Orenstein und
Koppel. For this purpose we used specially designed wedge that was used for wedging the
coal specimens. The wedge has lateral sides slopes of 17o to the longitudinal axis, a blunter tip
of width b = 5 mm and the wedge length l = 65 mm, figure 3.

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Figure 3. Schematic representation of the wedge and its position while testing
specific cut resistance in the primary method
At the original method the wedge for testing is being installed into a suitable hydraulic press
which is used for vertical force applying. The vertical force increases constantly until the
specimen fracture. During the tests, in addition to determining the value of the force which led
to the specimen fracture, it is being determined the size of the wedge penetration into the
speimen [2]. Value of specific line cut resistance KL and surface cutting force, KF are
calculated according to the following equations:
L
P
KL
(N/cmˊ) i
F
P
KF
(N/cm2),
where: P – breaking off force (N),
L – length of the wedge which was involved in the breaking (cmˊ),
F – specimens cross-section area (cm2).
Digging force testings for the underwater excavation purpose were performed with extended
wedge testing method which allows the same "in-situ" conditions which exist during the coal
excavation in this deposit. The influence of water and excavation depth are defined in terms
of external loads so that the sourroundig pressure value is constant and that it acts uniformly
in all directions, Figure 4.

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Figure 4. The position of the wedge during the
examination of the specific cutting resistance in
revised method
Figure 5. Hydrostatic pressure variation in water with
an increase in the depth at which excavation is carried
out
The values of water pressure for which it is necessary to carry out tests determined according
to equation:
hP 

,
where: γ – water unit weight, γ ≈10 [kN/m3],
h – excavation depth, h = 25 – 60 [m].
Pressure interval for digging force test is set according to the depths at which coal excavation
is performed, and it ranges from 25 to 60 m, and is:
2502510
min P
(kPa) ≈
50.2
min P
(bar),
6006010
max P
(kPa) ≈
00.6
max P
(bar).
Diagram of the pressure changes with increasing water depth, as well as the pressure interval
in which the digging force tests were conducted are shown in Figure 5.
5. TESTING APPARATUS
During the tests it was necessary to provide a water pressure p which exist at the coal
exploitation depth (h = 25 to 60 m), and accordingly the water pressures p during testing
were p = 250 to 600 kPa (p ≈ 2.50 to 6.00 bar). Given that the excavation is performed in
hydrostatic pressure conditions at a certain depth in the water, values of the principal stresses
are equal and their value is σ1 = σ2 = σ3 = p = 250 ÷ 600 kPa.
Digging force testings in coal underwater exploitation conditions were carried out using the
equipment for triaxial testing of soils that was modified to fulfill needs. The main task of
triaxial cell is to provide a sourrounding preasure and to maintain this pressure constant
during the entire course of the tests (σ3 = const), Figure 6. The same pressure conditions are
required for performing the digging force test in defined terms. In existing triaxial cell for the

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maximum specimen diameter of d = 150 mm is additionally installed wedge for digging force
testing according to the method that has been proposed by the company Orenstein und
Koppel, in Figure 3. The appearance of modified testing equipment is shown in Figure 7.
Figure 6. The basic principle and equipment for
triaxial test performance
Figure 7. The design of equipment suitable for
carrying out digging force testings in terms of the
underwater coal excavation
6. TESTING PROCEDURE
Digging force testings in underwater exploitation conditions were carried out on prismatic
specimens, which height was h = 100 mm, width a = 100 mm and length b = 100 mm
(diagonal of the base specimen was about 141 mm), which are placed at the center of the
triaxial cell base. During test the specimen was not isolated with rubber membrane, because
the material (coal) in natural conditions is also exposed to the water at a certain depth. After
installation of specimen and triaxial cell closure, cell was loaded with distilled water, and then
the load application was carried out in two phases. In the first stage sourrounding lateral load
σ3 was applied and it was kept constant during the entire course of the test (σ3 = const). In the
second phase, the axial load σ1 was gradually increased until fracture occures. The specimen
before and after examinations is shown in Figure 8.

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Specimen after tests
Specimen before tests
Figure 8. Specimen appearance before and after digging force tests
in underwater coal excavation conditions (sample labeled U-11)
The presented digging force testing procedure in underwater coal excavation conditions was
used for tests in the aquatic environment with a gradual increase of hydrostatic pressure,
regarding to lateral load was σ3 = 0,0 - 2.5 - 3.5 - 4.5 - 6.0 bar. Results of carried out
laboratory tests are shown in the Table 1.
Table 1. Test results of deggining force tests for all defined lateral pressures 3 (SAMPLE LABELED U-11)
Sample
tag
Specific cutting force
depending on the lateral pressure

3 (bar)
0.00
2.50
3.50
4.50
6.00
U – 11
KL (N/cm’)
873.46
936.33
963.52
990.71
1029.80
KF (N/cm2)
57.38
61.73
63.48
65.24
67.80
7. CONCLUSION
For the purpose of revitalizing the cutting wheel of vessel bucket wheel excavator "Kovin"
(UCW-450), it was necessary to make a appropriate geomechanical laboratory research with
an emphasis on cutting resistance testing in underwater exploitation conditions. Using a
gradual increase of the hydrostatic pressure during the test, the working conditions of coal
exploitation at different depths of the excavation were simulated. According to depths which
are used for coal excavation, the pressures of the water column of about 2.5 to 6 bar were
provided, and during the actual test lateral loads σ3 = 0,0; 2.5; 3.5; 4.5; 6.0 bar were
applied. Digging force tests in coal underwater exploitation conditions were carried out using
the equipment for triaxial testing of soil that was modified for these purposes. With quoted
equipment and depicted method it is possible to provide the water pressures which exists at
the certain coal excavation depth, so that the results of digging force tests can be considered
authentic.

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REFERENCES
[1] Kolonja B. et al. (2009). Glavni rudarski projekat eksploatacije uglja i otkrivke na podvodnom kopu
"Kovin" u nebranjenom delu polja "A", Rudarsko-geološki fakultet, Beograd
[2] Radojević J. (1992). Mehanika stena, Rudarsko-geološki fakultet, Beograd
[3] Radojević J. (1979). Optimizacija brina i uglova rezanja rotornim bagerima u odnosu na utrošenu energiju i
instalisanu snagu mašine, doktorska disertacija, Rudarsko-geološki fakultet, Beograd
[4] Gojković N., Ĉebašek V. et al. (1979). Definisanje geomehaniĉkih parametara u funkciji tehnološkog
procesa eksploatacije na rudniku Kovin – geomehaniĉka istraživanja za odreĊivanje otpora kopanja reznog
toĉka UCW-450, na bageru Kovin I, elaborat, Rudarsko-geološki fakultet, Beograd